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> the errno codes (EAGAIN etc) are provided only as a compatibility for 
> posix apps that test "errno". On windows, we use the WSA return values 
> from the api functions and WsaGetLastError().
> ...
> So, the proposed patch is not a change, it is merely reinforcing the 
> previous practice of prefering the native error codes over the 'errno' 
> emulation.
Except that Microsoft's C library also uses some of the non-WSA versions. For instance read() (or _read()) is documented to set errno to EBADF or EINVAL on error. So EBADF and EINVAL are just as "native" as WSAEBADF and WSAEINVAL.
It is also quite common for python's C code to do stuff like
 errno = EINVAL;
 PyErr_SetFromErrno(PyExc_OSError);
errnomap in Objects/exceptions.c is used to convert some OSError exceptions to subclasses like PermissionError. It shouldn't be hard to use it to also convert WSAEINVAL to EINVAL etc.